1. Field of the Invention
The invention relates to a method of establishing a feedthrough, and to a feedthrough structure in an implantable apparatus for stimulating living tissue, the feedthrough having the function of connecting an output of a stimulating pulse generator inside a housing of the apparatus to an electrode arrangement outside the housing, of the type wherein a first conductor and a second conductor, both connected to the stimulating pulse generator, are led through the wall of the housing without making electrical contact thereto, a connecting detachable means between the second conductor and the housing is provided, whereby the first conductor is connected to a first terminal of the electrode arrangement and whereby the second conductor is connected to a second terminal of the electrode arrangement and the connecting means is detached when the apparatus is in a bipolar working mode.
2. Description of the Prior Art
It is important for an implantable apparatus for stimulating living tissue, a pacemaker for example, that no body fluids are allowed to permeate the apparatus as this would result in a cessation of the function of the apparatus. Therefore, it is necessary for such apparatuses to be well encapsuled.
A pacemaker basically comprises two units - a stimulating pulse generator and an electrode arrangement. The stimulating pulse generator, whose function is to generate stimulating pulses adapted to optimize a patient's heart function, is well enclosed in a housing. The electrode arrangement, whose function is to lead and deliver the stimulating pulses to the heart, generally comprises a connector and an electrode lead, whose distal end is an electrode tip positioned in the heart and whose proximal end is removably fastened to the connector and at the same time close-fitting. The connector itself is well encapsuled with the exception of openings for the proximal end of the electrode lead and for the connection with the stimulating pulse generator.
It is when the stimulating pulse generator and the connector are to be brought together that a feedthrough is of use. It shall connect the two units electrically and fit closely between them. Such a pacemaker is described in German OS 2914034, for example.
A pacemaker can operate either in a unipolar mode or a bipolar mode. It is known to use identical stimulating pulse generators but different feedthroughs and electrode arrangements for the two operating modes. In the unipolar mode, a stimulating pulse is generated by the stimulating pulse generator and conducted from the stimulating pulse generator through a conductor in the feedthrough to the electrode arrangement and via the electrode arrangement out to the heart. From the heart, the stimulating pulse returns to the stimulating pulse generator via the housing, which is connected to the stimulating pulse generator via the feedthrough. In the bipolar mode, another feedthrough is used, which, in addition to from the above mentioned conductor, also comprises a second conductor to which the stimulating pulse generator and a ring electrode of the electrode arrangement are connected. A stimulating pulse generated by the stimulating pulse generator is conducted, as for the unipolar pacemaker, from the stimulating pulse generator via the feedthrough and the electrode lead to the heart, whereby the stimulating pulse returns to the stimulating pulse generator via the ring electrode.
An embodiment of a known feedthrough for a unipolar connection is shown in FIG. 1 and an embodiment of a known feedthrough for a bipolar connection is shown in FIG. 2.
Since the stimulating pulse generator is identical for both unipolar and bipolar pacemakers, the use of two different type of feedthroughs causes a number of technical production problems.
Two different types of feedthroughs have to be manufactured and stocked, and, furthermore, it will, after the mounting of the feedthroughs to the stimulating pulse generator, be necessary to keep double stores to separate the two types of semi-manufactured pacemakers from each other. The decision whether unipolar or bipolar pacemakers are to be manufactured must be taken at a comparatively early stage of the production, which results in an increased vulnerability to variations in the actual need of the two types of pacemakers.
A feedthrough is disclosed in U.S. Pat. No. 4,301,805 located in a pacemaker and which can be used in both unipolar and bipolar modes. A first conductor is connected to a stimulating pulse generator, led through a housing via a first feedthrough terminal and connected to a first terminal of an electrode arrangement. A second conductor is connected to the stimulating pulse generator, led through the housing via a second feedthrough terminal and connected to a connecting means. The connecting means is a bridging system comprising three contact blocks and a set screw which can connect two blocks to each other. The second conductor is connected to the middle block and the housing and a second terminal of the electrode arrangement are respectively connected to the outer blocks. In the unipolar mode, the set screw connects the second conductor to the housing and in the bipolar working mode, the set screw connects the second conductor to the second terminal.